Of the aromatic C8 isomers, including the three xylene isomers and ethylbenzene, para-xylene is of particularly high value since para-xylene is useful in the manufacture of terephthalic acid, used in synthetic fibers and resins. Refinery and chemical plant streams containing the aromatic C8 isomers typically contain, at thermodynamic equilibrium, only about 22-24 wt % para-xylene, based on the weight of the xylene isomers in the stream. Separation of para-xylene from the other C8 isomers requires superfractionation and/or multistage refrigeration steps and/or adsorptive separation, all of which are energy intensive.
One known method for producing para-xylene selectively involves the alkylation of toluene and/or benzene with methanol and/or dimethyl ether (DME) over a solid acid catalyst. Selectivities to para-xylene in excess of 90 wt % (based on total C8 aromatic product) have been reported by reacting toluene with methanol in the presence of a catalyst comprising a porous crystalline material, preferably a medium-pore zeolite and particularly ZSM-5, having a Diffusion Parameter for 2,2 dimethylbutane of about 0.1-15 sec−1 when measured at a temperature of 120° C. and a 2,2 dimethylbutane pressure of 60 torr (8 kPa).
Known processes suffer from the presence of undesirable by-products, particularly phenol, which act as free-radical reaction inhibitors in the subsequent oxidative conversion of para-xylene to terephthalic acid and therefore substantially increase the reaction time of the para-xylene oxidation.
A method of reducing or eliminating such free-radical inhibitors in the para-xylene stream would be beneficial. A method wherein the free-radical inhibitors are converted to free-radical initiators to promote the subsequent oxidation reaction would also be beneficial.